It is common scientific knowledge that fertility in mammals is the stage of life during which the ovaries and testes produce eggs and sperm, respectively. In the female gender this process involves the periodic development of ovarian follicles which contain the eggs. In the human species, for example, a pair of ovaries possesses an estimated 300,000 egg-bearing primordial follicles at the time of puberty. When an individual reaches sexual maturity, some of these follicles periodically develop and release their eggs (a phenomenon called ovulation). In all mammalian species that have been studied, the processes of follicular development and ovulation are regulated by two important hormones that are rhythmically secreted from the pituitary gland at the base of the brain. The active-hormones are follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These two gonadotropic hormones are secreted by pituitary cells when the pituitary gland itself is stimulated by gonadotropin-releasing hormone (GnRH), a small peptide hormone that is produced by nerve cells in a portion of the forebrain known as the hypothalamus. Thus, mammalian fertility is controlled by both nervous and hormonal activities (Espey, L. L., and I. A. BenHalim, Obstet. Gynecol. Clin. N. Amer. 17:275, 1990)
FSH and LH are large glycoprotein molecules that belong to a group of hormones collectively referred to as "gonadotropins" because they stimulate the gonads. When one or more ovarian follicles reaches maturity by the actions of these hormones, there is a surge in GnRH secretion from the hypothalamus, and this surge promotes a sharp increase in FSH and LH release from the pituitary gland. This sudden increase in gonadotropins stimulates the mature ovarian follicles to enter what is called the "ovulatory process". The ovulatory process requires approximately 10 to 40 hours, depending on the species of mammal. At the end of this metabolic process, the mature ovarian follicles rupture and release fertile eggs into the oviduct (Espey, L. L., and I. A. BenHalim, Obstet. Gynecol. Clin. N. Amer, 17:275, 1990)
If an egg is fertilized and the developing embryo becomes implanted in the uterus, special embryonic cells called trophoblasts begin secreting a hormone called chorionic gonadotropin (CG). (In humans this hormone is called human chorionic gonadotropin and is abbreviated as hCG.) CG circulates from the uterus back to the ovaries and stimulates any ruptured follicles (which are now called corpora lutea) to produce large amounts of the steroid hormone progesterone (Espey, L. L., and I. A. BenHalim, Obstet. Gynecol. Clin. N. Amer. 17:275, 1990).
Progesterone has two principal functions; (1) it maintains the uterine lining in the nutritive state that is required to support a developing embryo, and (2) it circulates back to the hypothalamus and the pituitary gland and inhibits further secretion of GnRH, FSH, and LH. This inhibitory action of progesterone prevents a second ovulatory process from occurring in a gravid animal. That is to say, a second pregnancy cannot occur "on top of" an existing pregnancy. Thus, progesterone functions as a natural contraceptive during pregnancy. This steroid hormone is also the basis of the oral contraceptive pills that are commonly used today. Synthetic progestins (along with some estrogens) are used in "the pill" to block ovulation by inhibiting the normal preovulatory surge in GnRH, FSH, and LH (Espey, L. L., and I. A. BenHalim, Obstet. Gynecol. Clin. N. Amer. 17:275, 1990).
At the time of menopause, the ovaries lose their capacity to produce significant amounts of progesterone and other sex steroids like estradiol. The reduction in these ovarian steroid hormones causes an abatement in the negative feedback action that they normally have on the hypothalamus and the pituitary gland. The rest-fit of this decline in inhibitory action is a significant increase in the secretion of FSH and LH from the pituitary glands of menopausal women. These two "menopausal gonadotropins" (i.e., FSH and LH) are filtered by the kidneys from the blood into the urine. Thus, human menopausal gonadotropin (hMG) is a urinary extract that contains substantial amounts of both FSH and LH, two gonadotropic hormones with similar origins and similar molecular configurations. Since menopausal urine contains a high concentration of hMG, extracts of human urine are the most common source of commercial preparations of hMG that are used to induce ovulation and fertility in women (Harlin, J., S. A. Khan, and E. Diczfalusy, Fertil. Steril. 46:1005, 1986; Cook, A. S., B. W. Webster, P. F. Terranova, and B. A. Keel, Fertil. Steril. 49:704, 1988; Corsan, G. H., and E. Kemmann, Fertil. Steril. 55:468, 1991).